The origin of this invention may be traced back to a 1963 proposal (J. L. Hughes, "Nature", May 1963) that experiments involving the focussing of intense laser beams could have a role to play in particle-antiparticle physics. To try and achieve such a goal it was necessary to increase laser peak powers substantially over those available in 1963 and three types of segmented solid state laser media were identified, namely:
(a) laser media segmented in a direction parallel to the direction of propagation of the laser beam;
(b) laser media segmented in a direction perpendicular to the direction of propagation of the laser beam;
(c) a mixture of types (a) and (b).
Lasers of type (a) are known as Phased Array lasers and were studied and abandoned in 1963 by the inventor at the Royal Radar Establishment, Malvern, UK only to be revived in 1979 (Hughes and Ghatah, "Applied Optics", July 1979) and detailed in U.S. patents currently classified.
Lasers of type (b) are known as slab lasers and were first proposed by the inventor in "Applied Optics" in August 1967 in the "Exponential Amplifier" configuration. However, subsequent patents (U.S. Pat. Nos. 4,039,962 and 4,132,955) describe the more practical format of the "Folded Exponential Amplifier" of which further development has led to the external cavity slab lasers of co-pending patents and the current application, the common laser development program being "Laser Radar" or more specifically "Variable Beamwidth Laser Radar" originally proposed by the inventor in 1963 and currently in classified patent formats in the United States Patent Office. From our early 1963-1964 field trials of Laser Radar, on Salisbury Plains, UK (Royal Radar Establishment Technical Memo, "Ruby Laser Field Trials", Hughes and Preece, July 1966) it was clear that visible and near infra-red laser wavelengths would not be suitable for target designation and ranging under adverse weather conditions and it would be essential to operate at longer wavelengths to overcome the scattering and absorption problems. The present invention utilizes a single slab of laser medium which duplicates as both a pulse generating medium and a pulse amplifying medium in an external cavity slab configuration as described in co-pending patent applications. By immersing the single slab oscillator-amplifier systems in a Raman shifting gas under pressure the invention's configuration automatically realises a folded path Raman shifter where a normal 100 cms path is reduced effectively to one of 10 cms. Using an Erbium doped yttrium aluminium garnet crystalline slab the fundamental wavelength emitted is 2,936 nanometers which can then be shifted via the Raman effect to the 10,000 nanometer region.
The invention has a wide range of applications other than target designation and ranging. This wider application range arises because if the invention incorporating a slab of neodymium doped yttrium aluminium garnet is used to generate a pulse at 1,064 nanometers, this can be shifted into the 1,100 to 1,500 nanometer wavelength range then frequency doubled or tripled into the visible portion of the electromagnetic spectrum. For example, if we Raman shift the Nd:YAG output of 1,064 nanometers to 1,280 nanometers then frequency double to 630 nanometers we have a source of laser light suitable for cancer radiation therapy. If we shift 1,064 nanometers to 1,154 nanometers and double to 577 nanometers, we have a very useful wavelength for the removal of birthmarks. On the other hand if we shift the 1,064 nanometers of Nd:YAG to 1,600 nanometers double to 800 nanometers and double again to 400 nanometers we have an effective operating wavelength in the deep blue. Alternatively, we could shift to 1,200 nanometers and triple the frequency to 400 nanometers. The number of combinations is very large.